The present invention relates to solid immersion lens (SIL) structures and in particular to techniques for constructing SIL structures, as well as selected applications of such structures.
Due to the limitations on resolutions obtainable with conventional optical lenses for applications such as microscopy, techniques have been developed to decrease the Rayleigh limit on transverse resolution xcex4. The Rayleigh limit is given by xcex4=0.82xcex/(NA) where xcex is the wavelength and NA is the numerical aperture of the focusing objective (NA=n sin (xcex8), where n is the refractive index of the medium, and xcex8 is the angle between the outermost rays focusing on the sample and the optical axis).
Coherent light such as laser light can be used to precisely control the wavelength of illumination xcex. One way to decrease the transverse resolution is to increase the index of refraction of the optical medium, such as by use of oil-immersion microscopy or use of a solid immersion lens.
If an SIL is placed in contact with the sample under examination, illumination can be more readily focused on it, and use of the high NA of the system allows efficient collection of the excitation light with high optical transmission efficiency and observation of the sample with a very high resolution. In most of the cases, the SIL is used primarily for near-field microscopy, where the air gap between the SIL and the sample oblige those who do not want to use evanescent waves to work with a NA smaller than one.
A problem with the SIL technology is the complexity of its manufacture. For example, a polished glass sphere provided with a sequence of progressively finer alumina powders, requires a polishing time typically of many hours. Furthermore, the result is not perfect, and the polished surface is slightly rounded. Moreover, known lens structures in SIL configurations involve objective lens sets that are self contained and thus are difficult to use in a manner that maintains the lens in immersion contact with the object under observation.
What is needed is a method for simple, inexpensive and rapid construction of a solid immersion lens and a lens structure which is suited for low-cost, even disposable usage.
According to the invention, a solid immersion lens structure is formed of an optically clear low temperature moldable material such an elastomer cast to a desired shape and smoothness in a pliant mold which has highly undercut margins. Further according to the invention, a method for construction of a solid immersion lens structure includes providing a pliant mold defining a lens-shaped cavity in which a solid immersion lens is cast, casting a liquid material into the lens cavity, permitting the liquid material to set to form the solid immersion lens portion and removing the solid immersion lens portion from the pliant mold with the highly undercut margins. A specific material for use as the solid immersion lens is a thermally-resilient deformable material such as optically-clear silicone elastomer of a refractive index n greater than 1.2 and preferably greater than 1.4, such as a room temperature vulcanization (RTV) elastomer, specifically General Electric RTV 615. Preferably, the mold itself may be constructed of this material and the SIL structure can be a rigid setting material. The SIL structures according to the invention may be a disposable lens element and/or a light collection element integrated with a specimen carrier of a microfabricated flow cytometer.
According to a specific embodiment of a method according to the invention, a first liquid elastomer such as RTV is injected into a container and allowed to solidify to a pliant elastomeric solid, then a small nonreactive bead of the shape of the desired lens (a sphere) is placed on the first layer then partially covered with a layer of the liquid elastomer of a controlled thickness less than the diameter of the bead and allowed to solidify to stable pliancy. Thereafter the shaping bead is removed to yield a pliant smooth-walled mold of maximum diameter d with highly undercut margins around an orifice. The mold and adjacent region are then treated with an oxygen plasma to create a nonreactive, nonbinding surface interface. Then a third layer of optically-clear liquid moldable material, such as an RTV elastomer, having a thickness slightly greater than the depth of the mold is injected into the mold and over the region around the orifice and then allowed to solidify. The resultant structure is peeled from the pliant second layer to yield a lens element in the form of a bead embedded on an attached flange, namely a solid immersion lens structure in accordance with the invention. The pliant second layer is reusable as a mold.
Further according to the invention, a method is provided for imaging an object using a low cost lens element in an SIL configuration. According to this method, an object to be observed, preferably immersed in fluid, is guided along a passage defined by an integrally molded-together body portion and a solid immersion lens portion, where the solid immersion lens portion is optically aligned with a position in the passage. The object is positioned in the passage in alignment with the solid immersion lens portion so that the object is within a field of view extending through the spherical solid immersion lens portion. The object, immersed in a fluid of high index of refraction, is viewed through the spherical solid immersion lens portion of an even higher index of refraction, and the object is imaged onto a viewing surface.
Further according to the invention, a method is provided for collecting light emissions with high efficiency through a low cost lens element in an SIL configuration. An object to be observed is immersed in fluid and positioned in alignment with the solid immersion lens portion so that the object is within a field of light collection extending through very large numerical aperture spherical solid immersion lens portion. The object, immersed in a fluid of high index of refraction, emits observable optical energy typically by fluorescence in response to excitation, and the emissions at selected wavelengths are collected through the spherical solid immersion lens portion of an even higher index of refraction and directed to a sensor, typically without imaging, so that the emissions can be measured. The structure admits to high collection efficiency.
Further according to the invention, a solid immersion lens structure comprises a solid immersion lens portion with highly undercut margins interfacing on a flange portion, together with and a body portion in which there is a cavity or passage for carrying an object or sample to be imaged or from which light is to be collected, where at least the solid immersion lens portion is of a molded material formed in a mold of pliant material with highly undercut margins.
The invention will be better understood by reference to the following detailed description and the accompanying diagrammatic drawings.